Short Answer:

Vapor pressure is defined as the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. It shows the tendency of a liquid to evaporate. The higher the vapor pressure, the faster the liquid evaporates.

In simple words, vapor pressure is the pressure created by the molecules that escape from the liquid surface into the vapor phase. It depends mainly on temperature — as temperature increases, vapor pressure also increases because more molecules gain enough energy to escape from the liquid surface.

Detailed Explanation :

Vapor Pressure

Vapor pressure is an important physical property of liquids and plays a significant role in fluid mechanics, thermodynamics, and many engineering applications. It refers to the pressure exerted by the vapor molecules when a liquid and its vapor are in thermodynamic equilibrium inside a closed container.

When a liquid is kept in a closed vessel, some of its surface molecules gain sufficient kinetic energy to escape into the vapor phase. As the number of vapor molecules increases, they start colliding with the liquid surface and some re-enter the liquid. Eventually, a balance is reached between the number of molecules leaving and returning — this condition is called dynamic equilibrium. The pressure exerted by the vapor at this stage is known as vapor pressure.

Concept of Vapor Pressure

Every liquid has a natural tendency to evaporate. This happens because the molecules of the liquid are constantly moving, and some of them possess enough energy to overcome the attractive forces holding them in the liquid phase. When these energetic molecules escape into the vapor phase, they exert pressure on the liquid surface.

If the liquid is placed in a closed container, this process continues until the number of molecules escaping into the vapor phase equals the number returning to the liquid. The system is then in equilibrium, and the vapor pressure becomes constant for a given temperature.

This vapor pressure depends entirely on the temperature and the nature of the liquid. Liquids with weak intermolecular forces, such as alcohol or ether, have high vapor pressures and evaporate easily. In contrast, liquids like water or mercury, which have strong intermolecular bonds, have low vapor pressures.

Units and Dimensions

• SI Unit: Pascal (Pa) or N/m²
• Other Common Units: millimeters of mercury (mmHg) or torr
• Dimensional Formula: M¹L⁻¹T⁻²

At standard atmospheric pressure (760 mmHg or 101.3 kPa), the temperature at which the vapor pressure of a liquid equals the atmospheric pressure is called its boiling point. Thus, boiling occurs when the vapor pressure of the liquid equals the external pressure.

Dependence on Temperature

Vapor pressure increases rapidly with temperature. As temperature rises, the molecules in the liquid gain more energy, increasing their tendency to escape into the vapor phase.

For example:

• At 20°C, the vapor pressure of water is about 2.34 kPa,
• At 100°C, it becomes 101.3 kPa (equal to atmospheric pressure), which is why water boils at this temperature.

This shows that temperature directly affects vapor pressure, and the relationship between them is exponential rather than linear.

Mathematical Expression

The relationship between vapor pressure and temperature can be expressed by the Clausius-Clapeyron equation:

or in integrated form:

where,

• P₁ and P₂ = vapor pressures at temperatures T₁ and T₂,
• L = latent heat of vaporization,
• R = gas constant.

This equation shows that vapor pressure depends exponentially on temperature, and even a small increase in temperature causes a large rise in vapor pressure.

Factors Affecting Vapor Pressure

1. Temperature:
   Vapor pressure increases with temperature because more molecules gain sufficient energy to escape from the liquid surface.
2. Nature of Liquid:
   Liquids with weak intermolecular forces (like ether or alcohol) have higher vapor pressures, while those with strong forces (like water or mercury) have lower vapor pressures.
3. External Pressure:
   Though vapor pressure itself is independent of external pressure, the boiling point depends on it. When external pressure decreases, boiling occurs at a lower temperature because the vapor pressure can equal the external pressure more easily.
4. Surface Area (in open systems):
   A greater surface area allows more molecules to escape, which can temporarily increase the evaporation rate, though equilibrium vapor pressure remains constant for a given temperature.

Engineering Importance of Vapor Pressure

Vapor pressure plays a vital role in many engineering applications and designs, especially in systems involving fluids and heat transfer.

1. Boiling and Condensation Processes:
   The design of boilers, condensers, and evaporators depends on understanding vapor pressure and temperature relationships.
2. Cavitation in Pumps and Turbines:
   Cavitation occurs when the local pressure in a liquid falls below its vapor pressure, causing vapor bubbles to form. These bubbles collapse violently, damaging pump or turbine blades. Therefore, engineers must consider vapor pressure when designing hydraulic systems.
3. Refrigeration and Air Conditioning:
   In refrigeration cycles, refrigerants operate between pressures defined by their vapor pressures at different temperatures. Proper selection ensures efficient cooling.
4. Fuel Systems:
   In engines, fuel vapor pressure affects fuel delivery and combustion. Low vapor pressure prevents vapor lock, while high vapor pressure ensures proper fuel atomization.
5. Storage of Volatile Liquids:
   Substances with high vapor pressures require tightly sealed containers to prevent loss through evaporation and to avoid fire or explosion risks.

Examples of Vapor Pressure Values

• Water (20°C): 2.34 kPa
• Ethanol (20°C): 5.95 kPa
• Diethyl Ether (20°C): 58.0 kPa
• Mercury (20°C): 0.00017 kPa

These examples show that ether evaporates easily due to high vapor pressure, while mercury evaporates very little because of its low vapor pressure.

Practical Applications

• In Nature: Evaporation from lakes, oceans, and soil occurs due to vapor pressure.
• In Industry: Drying, distillation, and cooling processes use vapor pressure differences for material separation and temperature control.
• In Everyday Life: Sweat evaporation cools the body because it occurs at the vapor pressure corresponding to body temperature.

Conclusion

In conclusion, vapor pressure is the pressure exerted by the vapor of a liquid in equilibrium with its liquid phase at a given temperature. It indicates the liquid’s tendency to evaporate and is primarily affected by temperature and intermolecular forces. Liquids with high vapor pressure evaporate easily, while those with low vapor pressure are more stable. Understanding vapor pressure is crucial in engineering applications like boiling, condensation, refrigeration, and hydraulic design, ensuring safe and efficient system operation.